The partial discharge defect characteristics of cable terminals are short, and the defect range is entangled with the external environment, making it difficult to accurately locate. It must be detected along with the temperature characteristics and pattern recognition characteristics. In this paper, using the advantages of ultrasonic infrared thermal imaging, a partial discharge defect detection method for cable terminals based on ultrasonic infrared thermal images is proposed. This method uses image gradient grayscale to collect an ultrasonic infrared thermal image of the partial discharge defect characteristics of a cable terminal, suppress the complex background of the collected image via an intelligent pattern recognition processing method, and delete large-area ground objects and surfaces contained in the image. Using the K-means clustering algorithm, the characteristic range of the suspected partial discharge defects is delineated, and the partial discharge defect range template is constructed. After matching the reference range, information on the temperature characteristics of the suspected partial discharge defect range is obtained to diagnose whether there are partial discharge defects in the cable terminal. The experimental results show that this method can effectively obtain the partial discharge defects of cable terminals. The average accuracy of detecting different types of partial discharge defects in cable terminals was as high as 98%, and the average missed detection rate was 1%.